Warning: Thought experiment ahead. I have no scientific/engineering proof whatsoever.

It strikes me that if there are combustion by-products making their way back out to the intake valve area due to partially open intake valves during the exhaust purge that the same by-products would be sucked back into the cylinder on the next downstroke due to the vacuum created.

But if the intake valve area was "wet" with condensed oil vapor, then the combustion by-products would have something to stick to and would not ALL be sucked back in.

If true, this sounds like a perfectly vicious circle that would account for all the buildup AND support both positions of our learned co-contributors.

Also if true the solution would be to avoid the condensed oil vapors in the first place.

Lastly a chemical analysis of the oil in the catch can versus that captured from the walls of an intake port would speak volumes as to the relative contribution of each "source".